The patent document SE 13476/68 (324 747) and its U.S. equivalent 3,613,804 describes a rotating drilling equipment comprising an axially fixed rod holder and an axially movable and rotating chuck which is arranged to hold and to rotate the drill rod. The rod holder co-operates with the chuck in order to hold the drill rod when exerting and inserting it to and from the drill hole. In order to operate the rod holder a common arrangement containing cup springs is used. The cup springs press clamping jaws towards the drill rod from opposite sides for holding the drill rod firmly when to alter chuck positions. When the rod holder is to be opened a hydraulic fluid pressure in a hydraulic cylinder is acting on the cup springs pressing them together whereby the firm grip is released.
The drawback of this arrangement is firstly that the considerable force required has to incorporate a large and heavy packet of cup springs. Secondly, the effect when using cup springs is that they are operating at their highest level of force and resistance which might be a drawback in terms of reliability and security. The only way to increase capacity is to use bigger and a higher number of cup springs.
A further drawback is that one of the existing jaws has a tendency to lie closely on the drill rod even if the rod clamps has been pushed apart from each other which causes unnecessary wear of the rod and jaw concerned.
Furthermore there is a drawback in the difficulties in controlling the force exerted by the cup springs in the spring package for the force that presses the jaws towards the drill rod.